Another Kepler data release is scheduled for February and once again there will probably be a flurry of blogs speculating if the mission has found the interstellar Holy Grail -- an Earth-sized planet in the balmy habitable zone about a sunlike star.

A term that's often kicked around is finding an Earth-analog. But what does that really mean? Does it imply something is living there? If so, the answer is very likely to be decades away, and full of uncertainty.

What makes Earth habitable?
The habitability of Earth depends on a lot more than just orbit location and its mass. Proponents of the Rare Earth hypothesis say that a long chain of unlikely events led to the emergence of complex life here. The Gaia hypothesis proposes Earth is nurtured and reshaped by life into a single mega-organism.

The flip side is the Medea hypothesis that says life is self-destructive and poisons a planet. For example, over 2 billion years ago blue green algae pumped out oxygen that was toxic to ancient bacteria. Microbes may have sucked enough greehouse gasses out of the atmosphere to trigger two "mother of all ice ages" at 2.3 billion years ago and again at 700 million years ago.

Based on current theories of planetary evolution, any extraterrestrials observing the solar system 4 billion years ago would have seen oceans on Venus, Earth, and Mars. Alien scientists would have cataloged all of them as potentially habitable.

Earth’s biggest advantage comes from having plate tectonics that stabilize the amount of carbon dioxide in the atmosphere that keeps Earth warm. Carbonic acid (essentially soda pop) dissolved in silicate rocks is transported to our oceans by rain. Sea life makes carbonate shells from this runoff. This makes carbonate sediments on the floor of the ocean that are subducted back into the Earth though plate tectonics. Volcanism recycles the carbonates into the atmosphere.

On Venus, hot water vapor rose high into the atmosphere where water molecules were split apart by the action of sunlight. The hydrogen escaped into space and the oxygen got locked into the surface rocks. With the ocean lost, Venus could not kick-start plate tectonics. Why? Because water-cools and lubricates the conveyor-belt motion between the low-density crustal plates and Earth’s plastic upper mantle. Therefore without water, moving the surface crust would be as hard as running a car engine without oil. Carbon dioxide is pumped into Venus’ atmosphere by episodic global volcanic outbursts, but never locked back into the crust. Hence Venus withers under a runaway greenhouse effect.

At one-tenth Earth’s mass, Mars was too small to hold onto an appreciable atmosphere. The oceans froze over when geologic activity died out. The carbon dioxide from volcanoes was chemically locked away in the rocks and never recycled.

Signs of life
What makes, me cautious about the discussion of “Earth-like” planets is that we can’t find extraterrestrial life in our own celestial backyard.

The problem is that if life exists elsewhere in the solar system it must be living underground on places like Mars, Europa, and Enceladus. And so detecting it from many million of miles away is nearly impossible. There have been reported observation of localized methane pockets on Mars that could come from subsurface microbes. But the observations are difficult to make and highly debated.

We measured anomalous activity in Martian soil experiments aboard the NASA Viking landers 35 years ago. The results are still being debated today as to whether it was a signature of biological metabolism. The same is true for the legendary Mars meteorite AHL 48001 that burst onto the scene in 1996 with suspected evidence it had microbial hitchhikers onboard.

The only convincing evidence for life on Mars will be when we have a microbe in the lab from a sample return mission. But we can’t do this for exoplanets. Evidence for life must all be deduced from remote sensing.

The first major step in this direction may come from the planned James Webb Space Telescope. If astronomers get lucky, Webb will be able to study a nearby super-Earth orbiting a red dwarf star. If Webb measures water vapor, it would suggest that oceans exist on the planet.

Is seeing believing?
But how would we convince ourselves anything is living on a Goldilocks planet?

For astrobiologists, life is defined as a chemical system that undergoes Darwinian evolution. That definition works in the lab where you can put matter under a microscope, says geoscientist James Kasting of Pennsylvania State University. But what about on the surfaces of chemically exotic worlds?

As far back as 1964 scientists, who were then called exobiologists, suggested that a planet’s atmosphere must be in extreme chemical disequilibrium if life is there. For example, aliens studying Earth from afar would note that oxygen and methane levels are 20 times greater than what would be expected if Earth were lifeless. But dead planets can be in disequilibrium too, it’s only a matter of degree, says Kasting. What's more, living planets can look like they are in chemical equilibrium, he adds.

For the chemistry of life to be remotely detected, we must find a planet where photosynthesis is is being conducted by surface organisms. So there must be surface water too — even if it is loaded with arsenic.

A very high excess of oxygen, methane, and nitrous oxides would certainly be suggestive of life. But the only way to declare the world inhabited would be to rule out all possible abiological explanations.

No doubt our modeling of exoplanet atmospheres is simplistic. And when the real data trickle in from something as big as an 18-meter aperture space telescope, astrobiologists will be befuddled interpreting it. They may never reach a consensus.

So my caution is that you’ll be reading again and again and about so-called “Earth-like” planets. But convincing ourselves of a place where something is actually living is very problematic. I predict that when the first science paper does come out claiming discovery of an extrasolar biosphere, there will once again be intense skepticism and allegations of hype.

The risk and reward in answering the question “are we alone?” is so high, it is the single biggest motivation for an extraterrestrial civilization to invest in interstellar travel (unless they just want to eat us). Their space program’s overarching goal would be to “seek out new life,” as TV’s Captain Kirk so succinctly put it 45 years ago.

But we have no evidence of such alien visitations. We haven't found extraterrestrial life, and apparently it hasn't found us.